1. Field of the Invention
The present invention relates to an improved process for the chlorination of phenolic compounds utilizing gaseous chlorine, which phenolic compound starting materials are substituted in both ortho positions with respect to the hydroxyl group.
2. Description of the Prior Art
Exemplary of an important phenolic compound which can be prepared by chlorination of an ortho-substituted phenolic compound is 2,4,6-trichlorophenol.
The usual process for the preparation of 2,4,6-trichlorophenol comprises the chlorination of 2,4-dichlorophenol.
However, a minor proportion of 2,4,5-trichlorophenol (on the order of 0.003 to 0.0010% of the weight of 2,4,6-trichlorophenol) is concomitantly formed. 2,4,6-Trichlorophenol, which is an intermediate in the synthesis of various other compounds, should be as pure as possible and should not contain any trace amounts of the 2,4,5-isomer.
One solution to this problem, therefore, would be to chlorinate 2,6-dichlorophenol, which would completely avoid the formation of 2,4,5-trichlorophenol. 2,3,6-Trichlorophenol, which can form in trace amounts in this particular instance, is far less problematical than 2,4,5-trichlorophenol.
Indeed, when 2,6-dichlorophenol is chlorinated with gaseous chlorine, it is found that the yield obtained is not particularly attractive. More specifically, a considerable amount of 2,4,5,6,6-pentachloro-2-cyclohexenone is formed, and this makes the reaction mixture highly unstable and difficult to purify.